1. Field of the Invention
The present invention relates generally to elevated temperature measuring apparatus and, more particularly, to such apparatus including a metallic thermocouple probe capable of withstanding temperatures in excess of 1650.degree. C.
Although the technology of refractory metal thermocouple probes has been well known for many years, the devices were not suitable for use in oxidizing environments due to rapid oxidation of sheath and thermocouple materials. The addition of a gas tight ceramic covering enveloping the thermocouple overcomes this limitations and allows use of such probes in an oxidizing environment such as within an operating gas turbine engine.
2. Background of the Invention
Thermocouple probes made of refractory metals such as tungsten and molybdenum have been used at temperatures up to 2315.degree. C. since the 1950s. The major problem with such probes was extremely rapid oxidation (occurring within minutes) of probes subjected to temperatures above 1200.degree. C. in the presence of any amount of oxygen in the surrounding environment. Therefore the probes were used only in applications where oxygen was not present, as in glass making, or was specifically excluded, as in certain foundry operations.
All manufacturers of refractory metal thermocouple probes state in their literature relating to that product line that the probes may be used at temperatures up to 2315.degree. C., but in every instance add the qualification that this capability never applies in oxidizing environments, even for short periods of time.
There have been attempts to "flame spray" ceramic coatings on sheaths, but the objective was to lower the temperature of the sheath. In these instances, the coatings were porous and were not designed to exclude oxygen.
A patent which is typical of the prior art is U.S. Pat. No. 4,977,001 to Greenspan which discloses a cladding for a molybdenum body such as a molybdenum tube intended to protect the molybdenum from corrosive high temperature environments. The cladding contains specified percentages by weight of molybdenum, aluminum, silicon, and alkali metals. While a process of manufacture is disclosed calling for formation of the cladding by bringing the molybdenum body into contact in the presence of oxygen with a refractory containing oxides of aluminum, silicon, and alkali metals at a temperature greater than 400.degree. C. there is no mention of the operating temperature for the molybdenum, although the specification acknowledges that temperatures greater than about 1700.degree. C. may cause a breakdown of the components in the protective cladding.
U.S. Pat. No. 4,989,992 to Piai discloses a probe equipped with a sensitive thermocouple element in contact with material to be measured, the thermocouple element being connected to electronic circuitry, distantly located, for amplifying and processing temperature signals and for displaying, recording, and controlling those signals. The probe comprises a sensitive thermocouple element constituted of two metallic wires having different thermoelectrical characteristics connected together at one end by a weld, the wires being embedded in a protective covering envelope of electrically and thermally insulating ceramic material, the weld between the wires being at a very short distance from the front surface of the envelope. Preferably, the wires of the sensitive element are constituted, respectively, of platinum and of platinum-rhodium with their ends welded together to form a hot junction of the thermoelectric couple. A temperature range mentioned in the patent for the measuring device therein disclosed is between approximately 20.degree. C. and 250.degree. C. This temperature range is far lower than that intended for the present invention and there is no mention nor apparent concern for protection of the thermocouple probe from oxidation.
A number of patents disclosing thermocouples with protective sheaths, although not airtight sheaths, are U.S. Pat. Nos. 4,871,263 to Wilson, 4,865,462 to Broomfield, and 4,721,534 to Phillippi et al.
It was in light of the foregoing state of the art that the present invention has been conceived and is now reduced to practice.